Adhesive bonds and polymeric coatings are commonly used in the assembly and finishing of manufactured goods. They are used in place of mechanical fasteners, such as screws, bolts and rivets, to provide bonds with reduced machining costs and greater adaptability in the manufacturing process. Adhesive bonds distribute stresses evenly, reduce the possibility of fatigue, and seal the joints from corrosive species. Similarly, polymer-based coatings are commonly applied to the exterior surface of manufactured products. These coatings provide protective layers that seal the surface from corrosive reactants, as well as provide a painted surface that can be aesthetically pleasing.
Among the best adhesives and coatings in terms of strength and durability are those based on thermosetting polymers. Typically applied as a liquid mixture of low molecular weight monomers, these adhesives wet and penetrate pores on the substrate surface. On cure, insoluble and infusible crosslinked polymers are formed; these are mechanically interlocked and often covalently bound to the substrate to which they are applied. Common amine-cured epoxies are a typical example of adhesives and coatings that employ thermosetting mixtures.
Although adhesive bonds offer many advantages over mechanical fasteners, they are essentially permanent. There are no methods available for ready disassembly of adhesively bonded objects. The separation strategies that do exist typically involve time-consuming chemical procedures requiring high temperatures and aggressive chemicals. Examples of such techniques are described in U.S. Pat. No. 4,171,240 to Wong and U.S. Pat. No. 4,729,797 to Linde et al. These techniques, although generally effective, are quite harsh and can damage the objects being separated, making them unsuitable for many applications.
Similarly, conventional coating materials, such as polyurethanes, epoxies, phenolics, melamines, and the like, are essentially permanent. Such coatings are often removed with an aggressive chemical agent that is applied to the coating surface to degrade the coating material. Mechanical abrasion, such as sand blasting or wire brushing, is also employed. Although these techniques are effective in removing the polymer coating, they are time and labor intensive, as well as being quite harsh and likely to cause damage to the underlying surface.
To provide materials that are more easily removed from a substrate, the prior art describes adhesives formed from reactive monomers containing linkages susceptible to chemical degradation, e.g., curable resins containing thermally labile linkages or thermally reversible crosslinks. Although these specially prepared materials are more readily cleaved from the substrate, they still require conditions that are harsh to delicate substrates or adjacent adhesive bonds.
Thus, there remains a need in the art for a material capable of being disbonded selectively and precisely under mild conditions, ideally from both bonded surfaces. Such a material would provide adhesive bonds and coatings that could be employed in a variety of applications where facile removal of the material from the surface is desired.